This invention relates to electromagnetic shields, and particularly, to radio frequency (RF) shields mounted on printed circuit boards.
It is often desirable to shield an area of printed circuit boards, or a volume associated therewith, with shielding members so that electromagnetic energy, e.g., radiated RF signals, are contained within the shielded volume or area, or excluded from the shielded volume or area. Such shielding is extensively used in television receivers, direct satellite broadcast receivers, radio receivers such as FM and shortwave, or portions of audio systems wherein low signal level circuitry is amenable to being effected by stray electromagnetic fields, e.g, 50-60 cycle fields emanating from AC power sources. Examples of circuits utilizing such shielding include RF tuner circuits which amplify low level signals received by an antenna, or low signal level audio circuits in the input stage of an audio system. Additionally, if a circuit generates a signal which can radiate either to other circuits of the same device or external of the device, the electromagnetic shielding will contain the radiation. Such a circuit can be an RF tuner local oscillator and/or a mixer.
The term "printed circuit board" as used herein refers to circuit boards having electrical conductors disposed on one or more sides of a dielectric board by deposition, etching or any other applicable process such as laminating (used for producing multi-layered boards). Modern television circuit board assemblies are generally manufactured by mounting components on the component side of the printed circuit board with their leads extending through apertures in the board to conductor pads of conductive pathways disposed on the conductor side of the board. The leads are typically soldered to the conductor pads. It should be noted that it is not uncommon for printed circuit boards to have components also mounted on the conductor side, or to have conductors on both sides of the board. RF shields typically have a shield member mounted on the component side by the extension of tabs or protrusions from the shield through apertures formed in the board to the conductor side. These tabs most commonly are soldered to a conductor pad proximal to or surrounding the aperture.
To lower production costs, it is desirable to have as many components and circuits as possible placed on a single printed circuit board to eliminate multiple handling of printed circuit boards, and to eliminate interconnection wiring between printed circuit boards. Towards this end, tuners are now being placed on the main printed circuit board of a television receiver. This is a change from tuners having their own printed circuit boards and located close to the input of their signals, e.g. antenna, or close to the tuning controls, such as channel selection switches. For the stand alone tuners, the boards were encased in their own shielding boxes which totally surrounded the printed circuit boards except for wiring terminals for interconnection of the printed circuit boards to other circuitry. It should be noted that both the components and the wiring between components, which are disposed within the shielded volume, are shielded from electromagnetic interference caused by extraneous signals or fields. With the location of the tuner on the main printed circuit board, the interconnection of the tuner with other circuitry is accomplished with the printed circuit wiring extending through the shielding to the outside of the shielded volume. However, in such a case, the degree of shielding of the tuner is reduced because both of the need to pass these printed circuit conductors through the shield, and the need to maintain sufficient structural integrity of the printed circuit board so that the shielded tuner portion of the board will be sufficiently supported.
The effectiveness of the shield assembly is greatly impaired if there are any "gaps" in the shielding. Such gaps, or unshielded areas, can permit substantial field leakage to or from the shielded volume. Although the shield itself completely encompass or circumpose an area on the printed circuit board, shield covers are necessary to shield the enclosed area of the printed circuit board in a direction normal to the board. In order to facilitate repairs, these shield covers are detachable from the shield. Even when the covers are in place, some leakage gaps along the locus of engagement between the shield and the shield cover occurs. The shield cover on the shield mounting side of the board (component side) usually has bent edges so that when that shield cover is mounted onto the shield, the upper edges of the shield are fully enclosed by the cover. This bent-over edge of the cover is often provided with spring-loaded protrusions to secure the cover to the shield while permitting removal.
However, enclosing the volume from the other side of the printed circuit board (the conductor side) is a more difficult matter. Elongated apertures or slots can be provided in the printed circuit board in order to facilitate mounting of a conductor side cover to the shield. This can be accomplished by the bottom shield having elongated protrusions extending through the printed circuit board slots to mechanically and electrically engage the sides of the shield in a manner similar to the top shield cover. However, this arrangement is not entirely satisfactory since it leaves unnecessary gaps in the shield assembly. Accordingly, it is desirable to provide a printed circuit board shielding arrangement which minimizes the leakage gaps.